As well known, in a wireless sensor network, positions of a plurality of mobile nodes are estimated using a few anchor nodes and utilized, for example, to track the positions of the mobile nodes and to provide service according to states of the mobile nodes.
In particular, a position estimation algorithm uses an angle and a distance between an anchor node and a mobile node, the intensity of a received signal, and the like. The distance is widely used as the most stable factor, and estimated from Time of Flight (TOF) that is, in turn, estimated using an ultrasonic or wireless signal. Obtaining exact distance information or TOF necessitates synchronization between the anchor node and the mobile node.
When a large-scale wireless network service such as mobile phone service is provided, supply of a network synchronized clock is highly critical for stable operation of a wide area system. The clock is commonly supplied to several anchor nodes, primarily, via a cable. On the other hand, in a temporary or small-scale network such as a sensor network or an ad hoc network, it is non-economic, troublesome, or impossible to install a separate network synchronization unit.
Meanwhile, in order to estimate a position of a mobile node in a conventional wireless sensor network, an Active Bat and Cricket system used a radio signal having a higher propagation speed (3×108 m/s) for synchronization and an ultrasonic signal having a lower propagation speed (about 340 m/s) for TOF calculation. Ubisense Ltd. supplied a separate wired-network synchronization signal from a control server for calculating a position to anchor nodes using a cable having a certain length.
IEEE802.15.4a suggested Two-Way Ranging (TWR) and Symmetric Double Sided-TWR (SDS-TWR) schemes capable of asynchronously calculating TOF without use of a network synchronized clock.
However, such TWR and SDS-TWR schemes still have an estimation error caused by frequency offsets of nodes, and require three or four times frame transmissions between one mobile node and each of anchor nodes, which increase the number of frames to be transmitted and a position estimation time. Accordingly, the TWR and SDS-TWR schemes are not suitable for serving a plurality of mobile nodes. For these reasons, IEEE802.15.4a suggested utilization of One-Way Ranging (OWR) following network synchronization, but implementation of network synchronization or the use of OWR are remained as development dependencies.